Immediately flush all pending deferred writes

Triggers an immediate flush of any queued deferred writes (when `deferred_writes.enabled` is true). Useful for operators to force persistence before maintenance or to clear backpressure. When deferred writes are disabled the endpoint returns a benign success with zero counts.

curl -X POST "https://athena-cluster.com/gateway/deferred/flush"

{
  "status": "error",
  "message": "string",
  "error": "string"
}